The present invention relates to a vitreous flux in which carbon is present and a process for making such a flux.
It is well known to apply a flux to the surface of a molten metal being cast. The flux is added to prevent oxidation of the melt, insulate the melt, lubricate the casting mold and remove deleterious materials (e.g., alumina) from the melt.
It is also known to include from 1 to 10 wt.% of powdered graphite in continuous casting flux products (see, e.g., U.S. Pat. Nos. 3,649,249 and 4,248,631) made from vitreous materials which have been ground to a particle such that at least 50% of the particles have a particle size less than 0.044 mm. The graphite is added to minimize heat loss from the surface of the molten metal. Graphite is easily mixed with powdery materials (e.g., finely ground vitreous materials); however, addition of such carbonaceous material to granular vitreous materials presents several practical problems. Premixing of the granular vitreous material and the carbonaceous material to form a flux in a location distant to the casting molds has been considered impractical because, during transport, the carbonaceous material settles and separates from the vitreous material. Mixing the carbonaceous powder with a material in the vicinity of the casting mold produces a carbon dust which may have a deleterious effect upon the metal being cast (particularly in the case of steel).
It is common practice to add carbon to a vitreous casting material shortly before the flux is applied to the molten mass. This approach is unsuitable with respect to a granular vitreous material which is to be used in an automated continuous casting process because, in such a continuous process, the flux material must be transported from storage to the caster. Such transporting of a mixture of granular vitreous particles and carbonaceous material results in a severe separation problem. While a sufficient degree of mixing might be maintained if the distance the flux were transported were sufficiently short, the carbon dust resulting from mixing in the vicinity could create problems in the metal being cast. Even if the degree of mixing could be preserved by adding excessive amounts of carbonaceous material, there is always the possibility that more than an acceptable amount of the unmixed excess would be added to the molten metal. Additionally, use of large amounts of carbon could result in the waste of an expensive starting material.
One approach to resolving these problems is that described in U.S. Pat. No. 4,248,631 (More et al.). More et al melts a silicate slag and then pours it into water to form a vitreous material. This vitreous material is then ground and screened to form relatively small particles. These particles are then superficially coated with carbon black and/or graphite with the aid of an adhesive material. This adhesive material is taught by More et al to be essential to achieve the appropriate degree of coating. In addition to increasing the cost of the product flux material, inclusion of such an adhesive material also makes it necessary to monitor the mixing operation to ensure that the slag material is exposed to enough adhesive to promote coating of the slag with an adequate amount of carbonaceous material. Care must also be taken during the mixing operation to prevent clumping of the carbonaceous material.
It would, therefore, be advantageous to have a carbon-containing flux material which could be easily produced at a location distant to the caster, stored (if necessary) and then transported to the caster either manually or automatically without significant separation of the carbonaceous material from the frit.